Stimulator

ABSTRACT

Apparatus for generating a sound field comprising a support, an elongated elastic column constructed to maintain its structural integrity when subjected to vibrational stresses, constraint structure constraining the column against transverse motion relative to the support while permitting longitudinal motion of portions of the column, an electromagnetic motor arranged to provide periodic longitudinal forces acting on an end of the column, the period of these forces being equal to the period of the longitudinal resonant vibrational mode of the column, a control system for controlling the amplitude of longitudinal motion of the column, and a piston affixed to an end of the column and having an extended gas-interacting surface transverse to the column.

United States Patent 1 Horsley 1 Jan. 2, 1973 [54] STIMULATOR [75] Inventor: Caperton B. Horsley, East Walpole,

3,249,913 5/1966 Smyth et al ..340/l0 Primary ExaminerBenjamin A. Borchelt Assistant Examiner-J. V. Doramus Attorney-W. R. Hulbert [57] ABSTRACT Apparatus for generating a sound field comprising a support, an elongated elastic column constructed to maintain its structural integrity when subjected to vibrational stresses, constraint structure constraining the column against transverse motion relative to the support while permitting longitudinal motion of portions of the column, an electromagnetic motor arranged to provide periodic longitudinal forces acting on an end of the column, the period of these forces being equal to the period of the longitudinal resonant vibrational mode of the column, a control system for controlling the amplitude of longitudinal motion of the column, and a piston affixed to an end of the column and having an extended gas-interacting surface transverse to the column.

17 Claims, 11 Drawing Figures PATENTED 2W5 3 708 033 SHEET 1 [IF 6 FIG I PATENTED 2|975 3.708.033

SHEET 2 or 6 Hllllll lllllllll PATENTEDJM 2 I975 SHEET 5 [IF 6 FIG 7 STIMULATOR This invention relates to generating sound fields.

It is a primary object of the invention to provide durable and reliable apparatus for efflciently producing sound of high intensity. Other objects are to feed large amounts of sound power to a resonant acoustic chamber used for agglomerating particles; and to use a resonant mechanical structure to assist in moving an air-interacting surface, thereby reducing the force required to move such surface.

The invention features apparatus for generating a sound field comprising a support, an elongated elastic column constructed to maintain its structural integrity when subjected to vibrational stresses, constraint structure constraining the column against transverse motion relative to the support while permitting longitudinal motion of portions of the column, an electromagnetic motor arranged to provide periodic longitudinal forces acting on an end of the column, the period of these forces being equal to the period of the longitudinal resonant vibrational mode of the column, a control system for controlling the amplitude of longitudinal motion of the column, and a piston affixed to an end of the column and having an extended gas-interacting surface transverse to the column.

In a preferred embodiment, the gas interacting surface is arranged to form a wall of a resonant acoustic chamber; the armature of the electromagnetic motor has a winding, connected to an alternating current source, running circumferentially around a longitudinal axis of the column, and a magnet provides a magnetic field radial in the winding (e.g., created by a circumferential electric winding in the magnet); the magnet has a plurality of lamina, preferably arranged in a multitude of segmented bundles, for conducting magnetic flux in regions adjacent the armature winding, the lamina being electrically insulated from one another along their laminated surfaces; the magnet is in the form of an annulus, encircling the column, and has an annular slot which receives the oscillating armature, the radial magnetic field extending across this slot; the column is rotationally symmetric about a longitudinal axis, and bilaterally symmetric about a transverse plane (i.e., the portion of the column on either side of the plane faces its mirror image across that plane), with the motor armature and piston secured at opposite ends of the column; where thecolumn is vertical, it is supported near a velocity minimum (e.g., at the above transverse plane of bilateral symmetry), eg, by a spring system; and, the constraint structure comprises a flexible member, which may be affixed to an end of the column (and, if conductive,- supply current to the armature), and is preferably a rectangular grid of flexible straps, each strap being secured at each end to the support and intermediately of its ends to the column.

Other objects, features, and advantages will appear from the following description of a preferred embodiment of the invention, taken together with the attached drawings thereof, in which:

FIG. 1 is a partly schematic view of a preferred embodiment of the present invention, with portions of the housing removed;

FIG. 2 is a longitudinal sectional view of the upper portion of the device FIG. 1 showing particularly the motor;

FIG. 3 is a longitudinal sectional view of the central and lower portions of the device showing particularly the air interacting piston and the central support;

FIG. 4 is a transverse section (at line 4-4 of FIG. 1) showing the central constraint structure;

FIG. 5 is a transverse cross section (at line 5-5 of FIG. 1) showing the lower constraint structure;

FIG. 6 is a transverse view of the magnet with portions cut away;

FIG. 7 is a transverse view of the device with the top removed and portions cut away to reveal the motor;

FIG. 8 is a view (from direction A in FIG. 7) ofa portion of the motor armature removed from the device;

FIG. 9 is a cross sectional view of the armature along line 9-9 of FIG. 8 with a portion broken away;

FIG. 10 is another view (from direction B in FIG. 7) of the motor armature removed from the device; and

FIG. 11 is a cross sectional view of the armature along line lll1 of FIG. 10.

FIG. 1 shows schematically an acoustic stimulator 10 including an oscillating assembly supported, by means discussed hereinafter, along the axis of cylindrical housing 22, a motor 12 including a magnet assembly 14 secured to housing 22 and an armature l6 fastened to the upper end of oscillating assembly 20,

and a piston assembly 90, fastened to the lower end of oscillating assembly 20 and having an extended surface 24 forming the upper wall of acoustic chamber 36. Housing cap 26 is bolted to the upper end of housing 22 and has affixed a support ring 30 and lock 28-, providing means for immobilizing the oscillating assembly 20 during shipment.

Referring to FIG. 1 and FIG. 3, oscillating assembly 20 consists of two identical spring tubes 42, 72; identical piston cone 66 and driver cone 80; four identical frusto-conical compression collars 70, 52, 71, 73; and spring tube coupling 46. As shown particularly in FIG. 3, lower spring tube 42, having the general form of a hollow cylinder, carries integrally on its proximal end 44 a flange 48 and an externally threaded portion 49. Spring tube coupling 46 carries integrally a shoulder 50 and a tapered, internally threaded lower portion54 and is situated so that a number of threads of portion 54 extend distally beyond shoulder 50. Threaded portion 49 of lower spring tube 42 is threaded into lower portion 54 of coupling 46 with threads of portion 49 extending proximally beyond the engaged threads and threads of portion 54 extending distally beyond the engaged threads. Compression collar 52 is compressed longitudinally between flange 48 and shoulder 50 during assembly to a degree such that the static tensile stresses in threaded portion 49 and threaded portion 54 exceed in magnitude the transitory stresses produced in said portions during oscillation. The distal end 62 of lower spring tube 42 similarly carries a flange 64 and an externally threaded portion 63. Piston cone 66, having the general form of a truncated cone, has a tapered internally threaded hub 65 and a shoulder 68, and is so situated that a number of threads of the hub 65 extend proximally beyond shoulder 68. I-Iub 65 is screwed onto threaded portion 63 with threads of hub 65 extending proximally beyond the engaged threads and threads of threaded portion 63 extending distally beyond the engaged threads. Compression collar is compressed between flange 64 and shoulder 68. Piston cone 66 has at its periphery an integral mounting ring 67 with 48 mounting holes 69 disposed equally around its circumference.

Upper spring tube 72 is identical to lower spring tube 42 and collar 71 is compressed between flange 81 and shoulder 74 of spring tube coupling 46, with threads of the upper threaded portion 82 of spring tube coupling 46 and threads of the lower threaded portion 83 of upper spring tube 72 extending respectively proximally and distally beyond the engaged threads. Driver cone 80 (FIG. 2) is identical to piston cone 66 and collar 73 is compressed between flange 84 and shoulder 85 of upper spring tube 72, with threads of threaded tube portion 86 extending proximally beyond the engaged threads and threads of threaded cone portion 88 extending distally beyond the engaged threads. All of collars 70, 71, and 73 are compressed such that static tensile stresses in the corresponding threaded portions will exceed in magnitude transitory stresses produced during oscillation.

Oscillating assembly 20 is made of stainless steel (e.g., E 4340 Aircraft Quality Steel). The mass of motor armature 16 is equal to the mass of piston assembly 90 which has a mass of about 60 lb. Spring tubes 42, 72 have central transverse cross sectional areas of 3 sq. in., and the resonant period of oscillating assembly 20 in its longitudinal vibrational mode is about 1/400 sec. The oscillating assembly is bilaterally symmetric about a transverse plane extending approximately across the junction of spring tubes 42 and 72, and is rotationally symmetric about its central longitudinal axis.

Bolted to spring tube coupling 46 is annular support bracket 120 and stepped mounting ring 122. Oscillating assembly 20 is supported on support bracket 120 resting on helical support spring 140, which is in turn supported by an apical portion 141 ofa conical bracket 142 bolted to extension 144 of housing 22. Two high durometer rubber bumper rings 146, 148 encircling support bracket 120 above and beneath bracket portion 141 prevent excessive flexing of support spring 140. Adjustment ring 150 is clamped onto mounting ring 122. As shown particularly in FIG. 4, the transverse position of oscillating assembly 20 is maintained by four flexible positioning straps 152, 154, 156, 158 bolted centrally to adjustment ring 150 and peripherally to housing 22. The lengthwise position as well as the tension of positioning strap 152 is adjustable during assembly by adjustment screw 162 at one end and adjustment screw 164 at the other end. Positioning straps 154, 156, and 158 are similarly adjustable. As shown particularly in FIG. 5, the transverse position of piston cone 66 is maintained by flexible positioning straps 180, 182, 184, 186 bolted centrally to mounting ring 67 and peripherally to brackets 188, 190, 192, 194 which are supported on housing 22. The lengthwise position and tension of positioning straps 180, 182, 184, 186 are adjustable in the same way as the lengthwise position and tension of strap 152.

As shown particularly in FIG. 7, the transverse position of driver cone 80 is maintained by positioning straps 195, 196, 197, 198 bolted centrally to driver cone 80 and peripherally to housing 22. Straps 197, 198 are made of material both strong and of good electrical conductivity such as copper-beryllium, and are isolated electrically from driver cone and from housing 22. Straps 152, 154, 156, 158, 180, 182, 184, 186, 195, and 196 are made, e.g., of stainless steel.

As shown particularly in FIG. 3, bolted to mounting ring 67 is piston assembly consisting of four truncated conical sheets 92, 93, 94, 95, and cylindrical sheet 96, all attached to junction 97, and five face pieces 101, 102, 103, 104, shaped to avoid buckling by large acoustic pressures, and forming extended surface 24. Face piece 101 has the general shape of a shallow dish and is affixed to and spans the interior of conical sheet 92. Dished annular face piece 102 is attached to and spans the interval between conical sheet 92 and conical sheet 93. Dished annular face piece 103 is attached to and spans the interval.between conical sheet 93 and cylindrical sheet 96. Dished annular face piece 104 is attached to and spans the interval between cylindrical sheet 96 and conical 94. Face piece 105 has the general shape ofa dished annulus and carries in addition a forward protruding lip 106. Face piece 105 is affixed to and spans the interval between conical sheet 94 and conical sheet 95. The outer cylindrical surface 107 (30 inch diameter in preferred embodiment) of face piece 105 fits snugly but movably (e.g., 0.030 to 0.040 in clearance) in orifice 32 of housing portion 34.

Motor magnet 14, shown particularly in FIGS.'2 and 6, includes annular magnet base 212 bolted at its periphery to housing 22. Annular magnet outer ring 214 and annular magnet inner ring 216 are bolted concentrically to magnet base 212. Annular coil 218 fits in the space between magnet outer ring 214 and magnet inner ring 216. The winding of coil 218 runs tangentially and is connected to leads 217. Coil 218 contains an annular slot 219. Outer annular pole piece 230 is bolted to magnet outer ring 214, and inner annular pole piece 240 is bolted to magnet inner ring 216 leaving an annular gap 261 between inner pole 240 and outer pole 230. Outer pole 230 consists of 12 segments 232 bolted to outer retainer 234. Each of the segments 232 is made up of lamina 236 bolted together and electrically insulated from one another with the short dimension of the lamina running approximately circumferential to the axis of motor magnet 14. Inner pole 240 is made up of eight segments 242 bolted to inner retainer 244 to form a generally annular structure. Each segment 242 is made up oflamina 246 bolted together with the short dimension of the lamina 246 running approximately circumferential to the axis of motor magnet 14. The lamina prevent interfering circumferential fields from being formed because of current flow in armature 16. Motor armature 16 supported by driver cone 80 fits with clearance within the gap 261 between outer pole 230 and inner pole 240 with its end protruding into the slot in coil 218. Magnet base 21.2, magnet outer ring 214, magnet inner ring 216, lamina 236 and 246 are made of ferromagnetic material suitable for conducting magnetic flux.

Referring to FIGS. 2, 8, 9, l0, and 11, armature 16 includes coil 270 circumferentially wound around an axis and clamped between ring shaped bottom insulator 272 and top insulator 274 by binding webs 276 distributed at equal intervals around armature 16. Each binding web 276 is attached (e.g., by silver soldering) to two pins 278 which pass through insulating ring 280 and insulating ring .282. All binding webs 276 and pins 278 are made of stainless steel except for connecting web 312 and associated pins 310 which are used for electrical connections and are made, e.g., of copperberyllium. Pins 278 are drawn up by nuts 284 bearing on ring 282. Armature 16 is bolted to driver cone 80 by bolts 290 passing through counter weight 292, driver piston 80, spacers 294, insulator ring 282, and insulator ring 280 into nuts 296. Electrical connections for coil 270 are made from terminal 302 to connector 304, to which is bolted strap 197 made of copper beryllium alloy which is bolted by insulated bolts 306 to armature 16. Clamped beneath and making electrical contact with strap 197 is one end of connector 308 which is clamped at its other end in contact with electrical conducting pins 310 made of copper beryllium and affixed to connecting web 312 made of copper beryllium. Web 312 makes electrical contact through connecting member 314 and spool-shaped member 316, which is snugly secured in aperture 317 of member 314, to the lower end of coil 270. The upper end of coil 270 communicates electrically with connector 320 made of copper beryllium which is in turn clamped in electrical contact with connector 322, again made of, e.g., copper beryllium. An end of connector 322 is clamped between insulator ring 292 and strap 198 making electrical contact with strap 198. Strap 198 is bolted peripherally to and makes contact with connector 324 (affixed to but insulated from housing 22) which is connected to terminal 326, (FIG. 7). Terminals 302, 326 are connected to the output of current source 356 (shown schematically), which uses conventional circuitry to provide an alternating current of controllable amplitude at a frequency controllable in a range including the resonant frequency of oscillating assembly 20. Amplitude control is achieved, e.g., by a conventional accelerometer 350 affixed to the central part of strap 196. The output of accelerometer 350 is conducted electrically through (copper-beryllium) strap 196 (which is insulated from armature l6 and from housing 22) thence to terminal 352 (attached to but insulated from housing 22), thence to a control input of current source 356. Bolts 306 and similar bolts used elsewhere on oscillating assembly 20 are preferably long shanked and tightened against collars 307 to produce significant elastic deformation (typically 0.003 to 0.009 inch) in shank.

In operation motor magnet 14 is energized by connecting leads 217 to a source of dc electric current, thereby producing a magnetic flux following a path through magnet base 212, magnet outer ring 214, outer pole 230, radially across gap 261 in which is situated armature 16, through inner pole 240 and magnet inner ring 216 back to magnet base 212. A magnetic flux radial in coil 270 of armature 16 is thus produced (in preferred embodiments, the field in this gap is 10,000 gauss).

The tangentially flowing ac current in armature coil 270 interacts with the radial magnetic flux in gap 261 between outer pole 230 and inner pole 240 to produce a reciprocating longitudinal force on coil 270. The reciprocating force is in turn transmitted to armature 16 and thence to driver cone 80 at the upper end of oscillating assembly 20. The frequency of the ac electrical source supplying armature 16 is adjusted to correspond to the fundamental resonant longitudinal vibration frequency of oscillating assembly 20 (about 400 hz, in the preferred embodiment), and when this adjustment has been made, a standing longitudinal vibration is set up in oscillating assembly 20. Because of the identical structure of the oscillating assembly on either side of its transverse median plane and because of the equal masses of the piston assembly bolted to the lower end of oscillating assembly 20 and of armature 16 bolted to the upper end of oscillating assembly 20, this standing vibration will be symmetrical about the median transverse plane of oscillating assembly 20, with a velocity minimum occuring at or near the median plane and a velocity maximum occuring at the upper and lower extremities of oscillating assembly 20. Support bracket being situated near a velocity minimum point will give a minimum of interference with the oscillator of oscillation assembly 20, while armature 16 and piston assembly 90, being situated near points of maximum velocity will have a strong interaction with respectively the magnetic field of magnet 14 and acoustic chamber 36.

When the amplitude of the oscillations of oscillating assembly 20 is low, accelerometer 350 senses this fact and acting through current source 356 causes the maximum design current to be supplied to coil 270. With maximum current in coil 270, the reciprocating forces produced feed oscillatory power into oscillating assembly 20 in excess of the power drained from assembly 20 by losses and by interaction of piston assembly 90 with the contiguous gas. The amplitude of oscillation in assembly 20 will therefore increase. As the amplitude increases both the power fed into and the power drained from assembly 20 will increase (because of the increased motion of the armature 16 and the piston assembly 90), but the power fed will continue to exceed the power drained, and the amplitude will continue to increase to values many times greater than the armature forces could produce in a non-resonant structure. When the amplitude rises to the design limit (in the preferred embodiment, acceleration of 800 g and excursion of 0.050 inch), accelerometer 350, sensing this condition, causes current source 356 to reduce the current supplied to coil 270 appropriately to limit the amplitude to the design value. The reduced current required to maintain the design value of amplitude will vary depending on conditions in acoustic chamber 36, the accuracy with which the electrical supply frequency matches the resonant frequency of oscillating assembly 20 and other factors.

Operating at design values, acoustic stimulator 10 may be utilized to produce a sonic field having up to about 1.3 psi peak pressure in closed acoustic chamber 36. The alternating gas velocity of this sonic field causes differential movement of small (say 3 micron diameter) and large (say 50 micron diameter) particles suspended in the gas filling chamber 36. The smaller particles therefore are caused to collide and coalesce with larger particles and thereby become more readily removable from suspension.

Other embodiments will occur to those skilled in the art.

What is claimed is:

1. Apparatus for generating a sound field comprising:

a support,

an elongated elastic column constructed to maintain its structural integrity when subjected to vibrational stresses,

a constraint structure constraining said column against transverse motion relative to said support while permitting longitudinal motion of portions of said column,

an electromagnetic motor arranged to provide periodic longitudinal forces acting on an end of said column, the period of said forces being equal to the period of the longitudinal resonant vibrational mode of said column,

a control system for controlling the amplitude of said longitudinal motion of said column, and

a piston affixed to an end of said column and having an extended gas-interacting surface transverse to said column, wherein said column is bilaterally symmetric about a transverse plane, a movable part of said motor is affixed to an end of said column opposed to said end at which said piston is affixed, and said part has a mass equal to the mass of said piston.

2. The device of claim 1 wherein said column is arranged with its longitudinal axis vertical, and is supported on said support near said transverse plane.

3. Apparatus for generating a sound field comprising an elongated oscillating assembly for longitudinal vibration at a predetermined frequency including an elastic column, a mass having a gas-interacting surface affixed to one end of said column and a second mass affixed to the other end of said column, said assembly being supported at a minimum velocity region between said masses by a flexible structure permitting longitudinal motion of said minimum velocity region, and motive means operatively connected to said assembly to apply periodic forces to said second mass at said frequency.

4. The apparatus of claim 3 wherein said motive means is an electromagnetic motor and said second mass is the armature thereof.

5. The apparatus of claim 4 including a resonant acoustic chamber, and wherein said gas-interacting surface is arranged to form a wall of said chamber for generating a sound field therein.

6. The apparatus of claim 5 wherein said frequency is about 400 HZ.

7. An acoustic stimulator for feeding sound power to a resonant acoustic chamber and maintaining a sound field therein at a frequency approximately 400 HZ and at a level exceeding 0.5 psi peak pressure comprising in combination:

a support an oscillating assembly having an elongated elastically extensible central column, a motor armature mass attached to one end of said elongated column, and a piston mass with a laterally extended gas-interacting surface attached to the other end of said column, said gas-interacting surface being arranged to form a wall of said acoustic chamber, said assembly having a resonant vibration mode wherein said masses move in linear opposition to alternatively compress and extend said elongated central column at a predetermined resonant frequency, I an electric mo or including said armature mass operatively connected to drive said assembly in said vibration mode,

a flexing support to support said assembly at a region of velocity minimum while permitting movement of said region,

a constraint structure to maintain the lateral position of said armature mass, and

a constraint structure to maintain the lateral position ofsaid piston mass.

8. The device of claim 7 wherein said motor comprises an armature affixed to an end of said column with a winding running circumferentially around an axis longitudinal to said column, and a magnet providing a magnetic field radial in said winding, and said winding is electrically connected to a source of alter nating current, whereby said armature is axially oscillated.

9. The device of claim 8 wherein said constraint structure comprises a flexible electrically conductive member extending between said support and one end of said column and electrically connected to said armature for connecting said armature to said alternating current source.

10. The device of claim8 wherein said magnet is of annular construction encircling said column, and is secured to said support.

11. The device of claim 10 wherein said magnet has a magnetic flux conducting portion in regions adjacent to said armature winding formed of a plurality of lamina, arranged with their short dimensions substantially circumferential to said axis, whereby interfering circumferential fields are prevented adjacent said armature.

12. The device of claim 11 wherein said plurality of lamina are disposed in a multitude of segmented bundles.

13. The device of claim 9 wherein said magnet comprises a winding connected to a source of electrical current running circumferential to said axis.

14. The device of claim 10 wherein said magnet includes spaced annular portions defining an annular slot therebetween, said magnetic field is radial in said slot between said annular portions, and said armature is located for axial movement in said slot.

15. The device of claim 7 in which said constraint structure comprises a flexible member affixed to said column and to said support.

16. The device of claim 15 in which said flexible member is connected to an end of said column.

17. The device of claim 7 wherein said constraint structure comprises a plurality of flexible straps arranged in a rectangular grid, each strap having its ends secured to said support and an intermediate portion connected to an end of said column. 

1. Apparatus for generating a sound field comprising: a support, an elongated elastic column constructed to maintain its structural integrity when subjected to vibrational stresses, a constraint structure constraining said column against transverse motion relative to said support while permitting longitudinal motion of portions of said column, an electromagnetic motor arranged to provide periodic longitudinal forces acting on an end of said column, the period of said forces being equal to the period of the longitudinal resonant vibrational mode of said column, a control system for controlling the amplitude of said longitudinal motion of said column, and a piston affixed to an end of said column and having an extended gas-interacting surface transverse to said column, wherein said column is bilaterally symmetric about a transverse plane, a movable part of said motor is affixed to an end of said column opposed to said end at which said piston is affixed, and said part has a mass equal to the mass of said piston.
 2. The device of claim 1 wherein said column is arranged with its longitudinal axis vertical, and is supported on said support near said transverse plane.
 3. Apparatus for generating a sound field comprising an elongated oscillating assembly for longitudinal vibration at a predetermined frequency including an elastic column, a mass having a gas-interacting surface affixed to one end of said column and a second mass affixed to the other end of said column, said assembly being supported at a minimum velocity region between said masses by a flexible structure permitting longitudinal motion of said minimum velocity region, and motive means operatively connected to said assembly to apply periodic forces to said second mass at said frequency.
 4. The apparatus of claim 3 wherein said motive means is an electromagnetic motor and said second mass is the armature thereof.
 5. The apparatus of claim 4 including a resonant acoustic chamber, and wherein said gas-interacting surface is arranged to form a wall of said chamber for generating a sound field therein.
 6. The apparatus of claim 5 wherein said frequency is about 400 HZ.
 7. An acoustic stimulator for feeding sound power to a resonant acoustic chamber and maintaining a sound field therein at a frequency approximately 400 HZ and at a level exceeding 0.5 psi peak pressure comprising in combination: a support an oscillating assembly having an elongated elastically extensible central column, a motor armature mass attached to one end of said elongated Column, and a piston mass with a laterally extended gas-interacting surface attached to the other end of said column, said gas-interacting surface being arranged to form a wall of said acoustic chamber, said assembly having a resonant vibration mode wherein said masses move in linear opposition to alternatively compress and extend said elongated central column at a predetermined resonant frequency, an electric motor including said armature mass operatively connected to drive said assembly in said vibration mode, a flexing support to support said assembly at a region of velocity minimum while permitting movement of said region, a constraint structure to maintain the lateral position of said armature mass, and a constraint structure to maintain the lateral position of said piston mass.
 8. The device of claim 7 wherein said motor comprises an armature affixed to an end of said column with a winding running circumferentially around an axis longitudinal to said column, and a magnet providing a magnetic field radial in said winding, and said winding is electrically connected to a source of alternating current, whereby said armature is axially oscillated.
 9. The device of claim 8 wherein said constraint structure comprises a flexible electrically conductive member extending between said support and one end of said column and electrically connected to said armature for connecting said armature to said alternating current source.
 10. The device of claim 8 wherein said magnet is of annular construction encircling said column, and is secured to said support.
 11. The device of claim 10 wherein said magnet has a magnetic flux conducting portion in regions adjacent to said armature winding formed of a plurality of lamina, arranged with their short dimensions substantially circumferential to said axis, whereby interfering circumferential fields are prevented adjacent said armature.
 12. The device of claim 11 wherein said plurality of lamina are disposed in a multitude of segmented bundles.
 13. The device of claim 9 wherein said magnet comprises a winding connected to a source of electrical current running circumferential to said axis.
 14. The device of claim 10 wherein said magnet includes spaced annular portions defining an annular slot therebetween, said magnetic field is radial in said slot between said annular portions, and said armature is located for axial movement in said slot.
 15. The device of claim 7 in which said constraint structure comprises a flexible member affixed to said column and to said support.
 16. The device of claim 15 in which said flexible member is connected to an end of said column.
 17. The device of claim 7 wherein said constraint structure comprises a plurality of flexible straps arranged in a rectangular grid, each strap having its ends secured to said support and an intermediate portion connected to an end of said column. 